As the stainless steel thin sheet products, there can be mentioned the 2B product, BA product and polished product specified by JIS. The commercial values of these stainless steel sheet products are determined by surface characteristics such as the gloss, rusting resistance, presence or absence or degree of flaws called "gold dust", peculiar to BA products, and the grindability, and accordingly, there is a need to improve these characteristics.
To satisfy this demand, there have heretofore been adopted a process in which a hot-rolled strip before cold rolling is annealed and pickled and the surfaces then ground to remove flaws (called "coil grinding"), a double rolling/annealing method, and a method in which heat streaks are formed at the cold-rolling step, but satisfactory results cannot be always obtained according to these methods.
The inventors carried out research with a view to developing a production process for obtaining stainless steel thin sheets having superior surface characteristics and clarified the causes of the forming of concavities and convexities on the surface of the product under various conditions, including slab-heating conditions and hot-rolling conditions, while omitting the coil grinding step of removing surface flaws by grinding the surfaces of a strip and searched for a means of eliminating these causes.
The technical problem concerning the surface characteristics of a stainless steel product is how to produce a product having a good gloss, high rusting resistance and superior grindability, and free of flaw called "gold dust". Our research found that the main causes of the degrading of these characteristics are "overlap" defects present on the surface of a cold-rolled material.
These "overlap" defects are caused by the following surface unevennesses or pitting before cold-rolling.
i) A concavity formed by intergranular corrosion caused at the pickling of a hot-rolled strip.
ii) An undulation present on the surface of a pickled material, which is generally called "surface roughness". iii) A grind grain left at the grinding of the surfaces of a pickled material.
Of these surface unevennesses or pitting, the concavity i) by intergranular corrosion can be prevented by the method of preventing the sensitization of the material or by the selection of appropriate composition for the pickling solution. In connection with the grind grain iii) left after the grinding, desirably the grind grain is made finer, but to prevent a formation of this unevenness, the coil grinding step should be omitted so that no grind grain is present.
The surface unevenness ii) called "surface roughness" has a large influence, because a large surface roughness results in a degradation of the surface properties of the product. As the means for reducing the surface roughness of the pickled material, there have been known a method in which the hardness of the material is increased at the mechanical descaling of the material (Japanese Examined Patent Publication No. 60-56768) and a method in which the surface roughness is reduced at the pickling step, as disclosed in Japanese Examined Patent Publication No. 61-38270 and Japanese Examined Patent Publication No. 49-16698.